Vacuum metalizing high tensile steel parts



1,890,135 I 1 whenMrrmznsqmen-rmsm STEEL PARTS Gist-565 @J iiifihk, Elisabetta; cant, assignhr to Anadite; me, South Gate, Califl, nomination of California No Drawing. Application Febiiiiii 19, 1958 Serial No. 716,046 4' claim." (Cl. 117-107 My invention relates to improvements in the protection ofhigh tensile strength steel parts, such as those used in aircraft construction, and more particularly to the vacuum metaliiing or such parts to provide a protective coating inf such" a rn an'ner as to completely eliminate hydrogen embri ttlem'e nt of the steel parts. N I

The c'oating of steel parts used in aircraft construction with rotectivenietals, more particularly with cadmium, has become a practical necessity in aircraft construction, especially for protecting and maintaining the life of such steel ar-ts, Such coatings, as far as known, have been appllediexclusively by electroplating the steel parts to be protected; I i g However; while electroplated cadmium coatings have provided protection, has been found that the electrolytic 1 deposition of cadinium causes hydrogen embrittl'ement of the steel, and this has become a very seriousproblem, especially with high strength steels with a 180K p.s.i. or higher. rirthemanurab nre of steel aircraft parts, their dihien'sicns are kept as low as possible because of the deof reducing the added weight. Therefore, it is extremely desirable; and in man cases essential, to use strength ste els has been soliied by depositing a protective.

metal coating on the high tensile strength'steel parts susacuurn chamher in which the protective metal v d and the resulting vapors condensed onthe surfaces of the steel parts.

The primary object of the present invention is, there'- fore, to 'pjro'vid'e afrnethod for producing high tensile steel parts, particularly for aircraft construction which are coated with a protective non-corrosive metal and are free of hydrogen'enibrittlenient. I I

i 'e electrolytic plating of steel aircraft parts,'as for I with cadmium, a system of specifications or sta'n I ards have been developed and promulgated in order to control the duality of the protective coating. It has been found that the high tensile strength steel parts, prodii'ced in accerda'nce with the present invention, meet the stan ards and specifications required for electrolytically coated parts and at the same time are entirely free 'of hydrogen embrittlement, so thatthe'ir strength as structural and operative members is unimpaired. v

The vacuum vaporiaation of metals for the coating of ohjecits to produce decorations and 'for other purposes is well known, and in accordance with the invention, the improved 'niethod iscarriedoutin a vacuum chamber of suitable s'iae, evacuated in accordance with known practide," by suspending the high tensile strength steel parts in the vacuum chamber and vaporizing the coating metal, as for example cadmium beneath the parts, so that the vapors flow over the suspended parts and condense on them.

e strength st'eels for parts which are sub (HI wri The steel parts to be processed according to the invention having a tensile strength of 180K p.s.i. and above are cleaned, if necessary, but without the'use of cathodic cleaning, acid pickling or cleaning-by any means involving release of hydrogen. Any scale, grease, oil or rust is removed andithe parts are sandblasted by light grit blasting with 180 mesh or finer grit. The sandblasted parts are then immediately racked and placed in a vacuum" chamber, and a' vacuum of 0.5 micron is developed in the chamber by suitable pumping equipment. The protective metal to'be vaporized is" then heated in the chamber in a position below the steel'parts'. A suflicient coating, as for example, .0005 inch of the vaporized metal, is attained-in from 15 to 35 minutes; When the parts are placed in the chamber at roomtemperature, the maximum temperature attained by the steel parts during the vacuum metalizing is approximately F.

After completing the desired coatin'gof the protective metal, the parts are removed from the vacuum chamber and are ready for use or for the application of the usual paint or organic coating, without baking or heat treatment. Where cadmium is used as the protective metal in the vacuum metalizing process, as is preferred, the parts removed from the chamber do not have a bright appearance but have a dull matte or satin finish of graywhite metallic luster. The vacuum metalized cadmium coatings vi/erefo'und to accept post processing chemical treatments and organic coatings conveniently used on electroplated parts in accordance with standard aircraft finishing systems.

Steel, aircraft parts coated with cadmium by vacuum deposition have been found to meet all the requirements with respect to adhesion, corrosion resistance, and other requirements specified by Federal Specification QQ-P- 416, for electroplated coatings. In a particular instance, low alloy steel parts heat treated to 260K p.s.i. (thousands of pounds per square inch) minimum ultimate tensile strength were coated with cadmium by vacuum vaporization and deposition to the extent of from .0003" to .0005" and found to be free of hydrogen embrittlement. The vacuum metaliziiig procedure produces no change in tensile strerigth of the parts.

Adhesicn tests were performed on sheet steel test specimens 1 by 4" and approximately 0.04" thick which were coated with cadmium by vacuum vapor'ization and deposition. Thes e specimens coated with from 0.0003" to ,0005" of cadmiu'm were bent through an angle of on a diameter equal to the thickness of the sheet and then examined at 4X magnification for e idence of separation of the cadmium from the steel sheet. No separations; were found in these tests. It was found that, even with successive coatings of cadmium applied in as ma as five c'ycles, there was no evidence of separation and nqevidenc'e o'f cdnt'act between the successive coats when ex r'nine'd under the microscope. The vacuum coatprocedure has been checked with respect to the evennest-bribe'coatingeja irregularly-shaped parts. A bolt, for example, was coated in four cycles without changing its 'po's'it'ion in the rack in the vacuum chamber. The shanlc of th'ebolt was found to have a coating of .0014 inchfand'almost an equivalent thickness was deposited in the thread. Keyways, slots and surfaces at right angles to each other receive uniform coatings in one application and good coats are thrown into holes that would remain uncoated in electrolytic processing.

The effectiveness of the process of protecting steel parts having 'atensil'e strength'a'b'o've 1 80K. p;s.i. against corrosion by applyin'g vacuum nietaliz'ed coatings thereto was demonstrated by testing cadmium coatings applied in this manner according to Specification QQP416. In

v this testthe parts were subjected to a salt spray exposure for 192 hours, according to the standard procedure.

Patented' June 9, 1959' There was no rusting of the surfaces of the parts, or even along the lines scribed through the coating. In a particular instance, a high heat treat steel fighter landing gear forging was vacuum coated with .0005" cadmium and then given the salt spray exposure test for 192 hours. There was no rusting or blistering of the coating. In general, it was found that vacuum metalized coatings stood up better in the salt spray corrosion test than the electroplated cadmium coatings.

The importance of the process in connection with the elimination of embrittlement in high tensile strength steels is shown in the following table giving the results of tensile tests on electrolytic cadmium coated specimens and vacuum mctalized cadmium coated specimens. The table shows the reduction of area measurements in each instance, in accordance with standard hydrogen embrittlement test methods used for many years.

Electroplated (60 a.s.f.6 min.) Vacuum Inetallzed Tensile Ultimate, K p.s.i.

Tensile Ultimate, K p.s.i.

Percent Reduction of Area Percent Reduction of Area Specimen Specimen The test specimens reported on in the above table were 1" gauge length, 0.250" diameter 4340 low alloy steel, heat treated to 260K p.s.i. minimum ultimate tensile strength. The thickness of coating of all specimens was .0003" to .0005" and the tensile strength measurements were taken within four hours after the coatings were applied. The electrolytically coated specimens were tested without baking.

Known types of apparatus may be employed for carrying out the improved process and the metal to be vaporized and deposited on the high tensile strength parts may be vaporized either directly by resistance heated tungsten wires or their equivalent or by placing the metal in small crucibles heated by resistance heating elements, the crucibles preferably being distributed across and lengthwise of the vacuum chamber below the rack on which the steel parts are assembled for coating. The racks are preferably rotatable or movable, or located in such a way that all surfaces of the parts may be coated. In carrying out the process with cadmium, substantially pure cadmium was employed in the crucibles, and in tests made on the coated parts, it was found that the coating consisted almost exclusively of pure cadmium with only traces of impurities.

Some of the parts coated with cadmium may be mentioned as examples. A high tensile strength hex-headed bolt showed the following dimensions. The head before sand blast measured .7435", after sand blast .7422 and after cadmium coating .7426. The shank had a diameter of .4360 before sand blast, .4355 after sand blast and .4359" after cadmium coating. Another part, a 4340 steel fitting having a tensile strength of 260K p.s.i. and having fiat, round tapering and other surfaces showed very similar dimension changes. For example, a shank part before sand blasting had a diameter of .4975", after sand blasting .4972, and after cadmium coating by vacuum metalizing .4975

While the invention has been described and illustrated particularly in connection with the vacuum deposition of cadmium on high tensile strength steel parts, it is to be understood that other metals and combinations of metals may be vacuum vaporized and deposited on the high tensile strength steel parts to provide the desired protective non-corrosive metal coating. Some of these metals and alloys are; aluminum, tin, zinc, tin and Cadmium, tin and zinc, tin-cadmium-zinc. Other metals which may be used alone or in various combinations are magnesium, chromium, nickel, copper and silver. In certain instances, the steel part may be coated with one of these vaporizable metals and then subsequently coated with a diiferent metal. For example, nickel or chromium may be applied over cadmium. However, since many aircraft structural parts are ultimately coated with organic coatings, the vaporized cadmium or its equivalent in a protective vaporizable coating metal may be employed as the basic adherent coating. Cadmium is particularly well suited for this purpose, since it is relatively inexpensive, light in weight and durable.

The process of the present invention not only provides an adherent, durable protective coating for high tensile strength steel parts, but the process of vacuum deposition of cadmitnn completely eliminates hydrogen embrittlement and provides such parts free from dangerous failure.

I claim:

1. The method of coating high tensile strength steel parts having tensile strengths in excess of K p.s.i., comprising enclosing the steel part to be coated in a vacuum zone, applying a vacuum in said zone of the order of 0.5 micron, vaporizing cadmium metal as a protective corrosion-resistant metal in said zone, condensing the resulting cadmium metal vapor directly on said steel part to provide a substantially uniform coating of from 0.0003 to 0.0005" on said part, and effecting the vaporization of the cadmium metal and the direct coating of the steel part in the vacuum zone under temperature and other conditions which leave the tensile strength of the cadmium coated steel part unchanged, thereby producing a high strength steel part with an adherent protective cadmium metal coating and which is free of hydrogen embrittlement.

2. The method of coating high tensile strength steel structural parts for aircraft having tensile strengths in excess of 180K p.s.i., comprising enclosing the steel part to be coated in a vacuum zone, applying a vacuum in said zone of the order of 0.5 micron, vaporizing substantially pure cadmium in said zone, condensing the resulting cadmium vapor directly on said steel part to provide an adherent cadmium metal coating of from 0.0003" to 0.0005 on said part, and efiecting the vaporization of the cadmium metal and the direct coating of the steel part in the vacuum zone under temperature and other conditions which leave the tensile strength of the cadmium coated steel part unchanged, thereby producing a high strength steel part with a protective cadmium coating and which is free of hydrogen embrittlement.

3. The method of preventing hydrogen embrittlement of high tensile strength steel aircraft parts free of hydrogen embrittlernent and having tensile strengths in excess of 180K psi and protecting such parts against corrosion, comprising enclosing the steel part free of hydrogen embrittlement in a vacuum zone, applying a vacuum in said zone of the order of 0.5 micron, vaporizing cadmium metal as a protective corrosion resistant metal in said zone, condensing the resulting cadmium metal vapor directly on said steel part to provide an adherent cadmium metal coating of from 0.003" to 0.0005 on said part, and effecting the vaporization of the cadmium and direct coating of the steel part in the vacuum zone under temperature and other conditions which leave the tensile strength and hydrogen embrittlement characteristics of the cadmium coated steel part unchanged, thereby producing an adherent high strength steel part with a protective metal coating and which is free of hydrogen embrittlement without reducing its tensile strength.

4. In the protection of high tensile strength steel aircraft parts against corrosion where such parts. have tensile strengths in excess of 180K p.s.i. and are free from hydrogen embrittlement, the method comprising enclosing such a steel part in a vacuum zone, applying a vacuum in said zone of the order of 0.5 micron, vaporizing cadmium metal as a protective corrosion resistant metal in said zone, condensing and depositing the resulting cadmium metal vapor directly on the surfaces of said steel part during a period of from 15 to 35 minutes to deposit on said surfaces an adherent coating of from 0.0003" to 0.0005", and heating said metal part in the vacuum zone by the condensation of the cadmium vapor 175 F., whereby said high tensile strength and hydrogen embrittlement characteristics of said steel part are unchanged.

References Cited in the file of this patent thereon to a temperature not exceeding approximately 10 2,726,179

UNITED STATES PATENTS UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,890,135 June 9, 1959 Gordon W. Jenkins It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the 'said Letters Patent should read as corrected below.

Column 2', line 28, for "conveniently" read conventionally column 4, line 63, for "0,003 read 0.0003

Signed and sealed this 13th day of October 1959,

iii??? ROBERT C. WATSON Commissioner of Patents KARL H. AXLINE Attesting Officer 

1. THE METHOD OF COATING HIGH TENSILE STRENGTH STEEL PARTS HAVING TENSILE STRENGTHS IN EXCESS OF 180K P.S.I., COMPRISING ENCLOSING THE STEEL PART TO BE COATED IN A VACUUM ZONE, APPLYING A VACUUM IN SAID ZONE OF THE ORDER OF 0.5 MICRON, VAPORIZING CADEMIUM METAL AS A PROTECTIVE CORROSION-RESISTANT METAL IN SAID ZONE, CONDENSING THE RESULTING CADMIUM METAL VAPOR DIRECTLY ON SAID STEEL PART TO PROVIDE A SUBSTANTIALLY UNIFORM COATING OF FROM 0.0003" TO 0.0005" IN SAID PART, AND EFFECTING THE VAPORIZATION OF THE CADMIUM METAL AND THE DIRECT COATING OF THE STEEL PART IN THE VACUUM ZONE UNDER TEMPERATURE ND OTHER CONDITIONS WHICH LEAVE THE TENSILE STRENGTH OFTHE CADMIUM COATED STEEL PART UNCHANGED, THEREBY PRODUCING A HIGH STRENGTH STEEL PART WITH AN ADHERENT PROTECTIVE CADMIUM METAL COATING AND WHICH IS FREE OF HYDROGEN EMBRITTLEMENT. 